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Moons of Neptune
(top) and Triton (bottom), 3 days after the Voyager 2 flyby.]] Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself. It took about one hundred years to discover the second natural satellite, Nereid. Triton is massive enough to have achieved hydrostatic equilibrium, and would be considered a dwarf planet if it were in direct orbit about the Sun. Triton has a very unusual orbit that is circular but retrograde and inclined. Inward of Triton are six regular satellites, which all have prograde orbits that are not greatly inclined with respect to Neptune's equatorial plane. Some of these orbit among Neptune's rings. Neptune also has six outer irregular satellites, including Nereid, whose orbits are much farther from Neptune, have high inclinations, and are mixed between prograde and retrograde. Two natural satellites discovered in 2002 and 2003, Psamathe and Neso, have the largest orbits of any natural satellites discovered in the Solar system to date. They take 25 years to orbit Neptune at an average of 125 times the distance between Earth and the Moon. Neptune has the largest Hill sphere in the solar system, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons. Neptune's moons are named for aquatic personages in Greek and Roman mythology, many of them after Nereids, in keeping with Neptune's position as god of the sea. Discovery of Triton]] Triton was discovered by William Lassell in 1846, seventeen days after Neptune was discovered. Nereid was discovered by Gerard P. Kuiper in 1949. In 1981 Larissa was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Thole. No further moons were found until Voyager 2 flew by Neptune in 1989. Voyager 2 recovered Larissa and discovered five new inner moons, bringing the total of known moons of Neptune to eight. In 2002 and 2003 telescopic surveys found five additional outer moons from Earth observatories bring the total to thirteen. The five new outer moons are Halimede, Sao, Psamathe, Laomedeia, and Neso. Names Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa. See also Name conflicts of solar system objects. Note that Triton did not have an official name until the twentieth century. The name "Triton" was suggested by Camille Flammarion in his 1880 book Astronomie Populaire, but it did not come into common use until at least the 1930s. Until this time it was usually simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949). Characteristics Triton Irregular moons The diagram illustrates the orbits of Neptune’s irregular moons discovered so far. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~116 Gm for Neptune). Given the similarity of their orbits, it was suggested that Neso and Psamathe could have a common origin in the break-up of a larger moon.Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint) Triton, the biggest moon following a retrograde but a quasi-circular orbit, also conjectured to be a captured satellite, is not shown. Nereid, which has a prograde but very eccentric orbit, is believed to have been scattered during Triton's capture.Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504. Formation The mass distribution of the Neptunian moons is the most lopsided of any planet. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent (see diagram). This may be because the capture of Triton destroyed much of the original Neptunian system. It is likely that Neptune's inner satellites are not the original bodies that formed with Neptune but accreted rubble from the havoc that was wreaked after Triton's capture. Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites . The mechanism of the Triton’s capture has been the subject of several theories over the years. The most recent as of 2006 postulates that Triton was captured in a three-body encounter. In this scenario, Triton is the surviving member of a binary objectBinary objects, objects with moons such as the Pluto–Charon system, are quite common among the larger Trans-Neptunian objects. disrupted by its encounter with Neptune.C.B. Agnor & D.P. Hamilton Neptune's capture of its moon Triton in a binary-planet gravitational encounter, Nature, 441 (2006), pp. 192. (pdf). Numerical simulations show that a moon discovered in 2002, Halimede, has had a high probability of colliding with Nereid during the lifespan of the system.M.Holman, JJ Kavelaars, B.Gladman, T.Grav, W.Fraser, D.Milisavljevic, P.Nicholson, J.Burns, V.Carruba, J-M.Petit, P.Rousselot, O.Mousis, B.Marsden, R.Jacobson Discovery of five irregular moons of Neptune, Nature, 430 (2004), pp. 865-867. Final preprint(pdf) As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint) Table The Neptunian moons are listed here by orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a spheroid, but is comparable in size to our own moon, is highlighted in purple. Irregular (captured) moons are shown in grey; prograde in light grey and retrograde in dark grey. (Triton is also thought to be captured.) Notes References External links * Neptune's Moons by NASA's Solar System Exploration Category:Moons of Neptune Category:Lists of moons be-x-old:Спадарожнікі Нэптуна bs:Neptunovi prirodni sateliti bg:Естествени спътници на Нептун ca:Satèl·lits de Neptú cs:Měsíce Neptunu da:Neptuns måner de:Liste der Neptunmonde el:Δορυφόροι του Ποσειδώνα es:Satélites de Neptuno eo:Naturaj satelitoj de Neptuno fr:Satellites naturels de Neptune ko:해왕성의 위성 hr:Neptunovi prirodni sateliti it:Satelliti naturali di Nettuno lv:Neptūna pavadoņi lt:Neptūno palydovai hu:A Neptunusz holdjai nah:Tlāloccītlalli īmētz ja:海王星の衛星と環 no:Neptuns måner nn:Neptunmånane nds:List von de Neptun-Maanden pl:Księżyce Neptuna ru:Спутники Нептуна simple:List of Neptune's moons sk:Mesiace Neptúna sl:Neptunovi naravni sateliti fi:Neptunuksen kuut sv:Neptunus naturliga satelliter tr:Neptün'ün Ayları zh:海王星的卫星